WASHINGTON — Veterans who live as little as a 30-minute drive from a Veterans Affairs health care facility will instead be able to choose private care, the most significant change in rules released Wednesday as part of the Trump administration’s effort to fix years-old problems with the health system.

Veterans who can prove they must drive for at least 30 minutes to a Department of Veterans Affairs facility will be allowed to seek primary care and mental health services outside the department’s system. Current law lets veterans use a private health care provider if they must travel 40 miles or more to a V.A. clinic. Measuring commuting time rather than distance will greatly open the private sector to veterans in rural and high-traffic urban areas.

Supporters say the new policy, which is likely to go into effect in June, will help veterans get faster and better care. But critics fear it will prompt the erosion of the largest integrated health care system in the country as billions of dollars are redirected to private care.

The goal of the new policy, officials say, is to provide veterans with easier, streamlined access to health care.

“This is the most transformative piece of legislation since the G.I. Bill,” Robert L. Wilkie, the secretary of veterans affairs, said in a telephone interview this week. “It gets us on the road to becoming a 21st-century health care institution.”

Current law lets veterans facing a wait of 30 days or more for an appointment at their closest V.A. facility seek private care, but under the new policy, that would be reduced to 20 days, and with the goal of 14, by 2020. Veterans will also be allowed access to walk-in clinics; however, those will require co-pays for treatment after a third visit. If seeking a specialist after the new policy takes effect, veterans must prove a drive of at least 60 minutes.

Taken together, the percentage of veterans eligible for what officials refer to as “community care” currently — roughly 8 percent of the 7 million treated annually — would rise to between 20 and 30 percent, according to Department of Veterans Affairs officials.

Lawmakers and veterans advocacy groups — which have been wary of large-scale moves into the private sector — will be briefed about the program on Wednesday.

In recent years, Veterans Affairs hospitals have struggled to keep up with patient loads as service members returning from Iraq and Afghanistan — many with complex injuries and post-traumatic stress — hit the system at the same time that aging and increasingly ill older veterans made more use of it.

A scandal in 2014 over hidden waiting lists at V.A. facilities sent lawmakers in search of solutions, with many Republicans favoring more use of the private sector and Democrats preferring to add doctors and medical centers to the government-run system.

Congressional Republicans and the Trump administration have been greatly influenced by Concerned Veterans for America, an advocacy group with ties to the billionaire industrialist brothers Charles G. and David H. Koch, which has long championed expanding the use of private health care for veterans. Traditional veterans service organizations, which have largely opposed these changes, have had less say their chair at the table reduced to more of a stool under Mr. Wilkie.

The legislation passed last spring and signed by President Trump in June, the Mission Act, increased funding for the Department of Veterans Affairs and earmarked more money for private care. It is up to Congress to beef up both pots of money each year.

Critics fear that private health care, which tends to have higher costs than government-provided care, will force the department to cut corners elsewhere.

A congressionally mandated report in 2016, by a panel called the Commission on Care, analyzed the cost of sending more veterans into the community for treatment and warned that unfettered access could cost well over $100 billion each year. That same commission found quality of care at the V.A. to be very high, one area of agreement between V.A. officials and those who use and advocate the system.

The new standards were developed after V.A. officials studied both the military’s insurance plan, Tricare Prime, which sets a lower bar for access to private care than the department has historically had, and the Medicare Advantage program, which allows Medicare beneficiaries to buy private health insurance plans instead of using government-run fee-for-service Medicare.

The Department of Veterans Affairs will remain at the center of care coordination, and the private providers — who would be paid by the department at rates roughly comparable to the Medicare program — would not be permitted to cherry-pick the healthiest patients, V.A. officials said. About 26 percent of veterans pay a co-payment, and they would have similar co-payments at private doctors.

Department officials — including Mr. Wilkie — have repeatedly insisted that the department should and probably will remain the provider of choice for most veterans, who prefer the culture of a V.A. hospital to that of the private sector. But a shrinking veteran population over all in the United States and more reliance on private providers could lead to the closings of some government hospitals, some veterans groups and members of Congress warn.

Mr. Wilkie insisted that was not the goal of the new policy, and said that fears of full privatization were unfounded.

“I think it’s simple: People don’t want change,” he said of such concerns. “That is a normal human reaction.”